The palaeoecological and palaeo-oceanographical significance of ammonite mass-occurrences in the Alpine Lower Cretaceous

At present there is a total lack of information about Lower Cretaceous ammonite mass-occurrences in the Northern Calcareous Alps and no publications cover these phenomena. Projects recently carried out at the Institute of Palaeontology (University of Vienna) discovered two Lower Cretaceous mass--occurrences of ammonites in the Temberg Nappe of the Northern Calcareous Alps (Upper Austria). These could be of tremendous importance for the geological and palaeo-oceanographic interpretation of this northernmost tectonic unit of the Northern Calcareous Alps. The general aim of the present project is to solve these problems. Both occurrences are of different stratigraphical age in the Lower Cretaceous: The older one with dominating Olcostephanus (Olcostephanus) sayni KILIAN is from the Upper Valanginian, the younger one dominated by Karsteniceras cf. pumilum UHLIG is of Barremian age. A basic systematic-taxonomic study should throw light on the palaeobiology, palaeoecology, taphonomy and palaeo-oceanography of these two mass-occurrences and furthermore on ammonite mass-occurrences in general. The search for additional ammonite mass-occurrences of different stratigraphical levels in the Lower Cretaceous of the Northern Calcareous Alps and adjacent regions should reveal their lateral extent and whether or not they were deposited diachronously. The expected results will provide clues for deciphering the palaeogeographic and tectonic development of the Northern Calcareous Alps and geodynamic processes operating in the Lower Cretaceous. The innovative starting point of the project will enable models of genesis and emplacement of ammonite mass- occurrences to be developed.

The finished project mainly deals with palaeobiological, palaeogeographic and biostratigraphic aspects of ammonite mass-occurrences. Two Lower Cretaceous mass-occurrences in the Ternberg Nappe of the Northern Calcareous Alps (Upper Austria) have been investigated in the framework of the FWF project P13641-Geo (1999-2002) and were described for the first time. Both occurrences are of different stratigraphic age in the Lower Cretaceous: The older one (KB1-A) with dominating Olcostephanus (Olcostephanus) guebhardi morph. querolensis BULOT is from the Saynoceras verrucosum Zone (Late Valanginian), the younger one (KB1-B) dominated by Karsteniceras ternbergense nov. sp. is of Early Barremian age (Moutoniceras moutonianum Zone). The mass-occurrence of Olcostephanus (Olcostephanus) guebhardi morph. querolensis over an interval of almost 3 meters sediment thickness during the verrucosum Zone is interpreted as a long-term, `normal` accumulation from the water column during a particular time interval of the Upper Valanginian, mixed with layers of enhanced shell reorientation and transport. About 200 specimens of Olcostephanus, measuring between 1 and 15 cm in diameter, were investigated. The ammonite-fauna comprises 9 different genera with 10 species. The mixed autochthonous/allochthonous ammonoid assemblage of the Olcostephanus mass-occurrence shows some similarities to a `Kondensat Lagerstätte`. The Karsteniceras mass-occurrence over a sediment thickness of about 30 centimeters comprises 8 genera, each apparently represented by a single species, of which 4 are identified to species level. One new species - Karsteniceras ternbergense nov. sp. - is established. About 300 specimens of Karsteniceras between 0.5 and 3.5 cm in diameter were investigated. The geochemical results indicate that this mass-occurrence within the described Lower Cretaceous succession was deposited under intermittent oxygen-depleted conditions, associated with stable salinity-stratified water masses. The rhythmicity of laminated black shale layers and white bioturbated, organic- depleted limestones suggests that oxic and dysoxic conditions changed episodically. Karsteniceras inhabited areas of water stagnation, with low dissolved oxygen, showing peak abundance through times of oxygen depletion; these conditions hindered other invertebrates from settling such environments. The autochthonous Karsteniceras mass- occurrence can definitely be assigned to the deposition-type of a `Konservat Lagerstätte`. The Olostephanus mass-occurrence is most probably a more local phenomenon, whereas other investigations indicate that the Karsteniceras mass-occurrence is more widespread; this is supported by an additionally dicovered occurrence at Sparbach (Lunz Nappe, near Vienna).